The various embodiments described herein generally relate to implantable leads, and more particularly to MRI-safe implantable leads having high impedance electrodes.
An implantable medical device is implanted in a patient to, among other things, monitor electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g. tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g. cardiac pacing) to return the heart to its normal sinus rhythm.
In general, the IMD includes a battery and electronic circuitry, such as a pulse generator and/or a processor module, that are hermetically sealed within a housing (generally referred to as the “can”). An implantable lead interconnects the IMD and the heart. The lead generally includes a pacing electrode and at least one sensing electrode at a tip of the lead. Electrical signals are transmitted between the electrodes and the pulse generator. For an IMD, functional implant life time is, in part, determined by the energy delivered per pulse. The IMD will have a longer life if the energy delivered per pulse can be maintained at a minimum. Designs of the lead and of the electrodes which are used with the lead are influenced by the electrical signal required for pacing stimulation. Physiologically, the IMD should be capable of generating a signal with a sufficient magnitude to depolarize the excitable cells of the myocardium to initiate contraction. The electrode shape, size, surface area, material and impedance combine to determine the energy required of the IMD.
When patients implanted with IMD's are subjected to external electromagnetic interference, undesirable electric current and voltage could be induced by such interference and could create undesirable physiological effects, such as fibrillation and pain. Examples of IMD malfunctions have been traced to medical procedures, such as radiofrequency catheter ablation, electrocautary, dental procedures, magnetic resonance imaging (MRI) techniques, as well as other medical procedures. Of these, the MRI system is perhaps the most common.
MRI is a technique that provides a non-invasive method for the examination of the internal anatomy of a human body. This provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities. However, it may be unsafe and even hazardous to place patients implanted with IMD's through the MRI system because of the high radiofrequency (RF) field that is generated. The high RF field may cause heating of the conductive components of the IMD, such as the housing, the lead, and the electrodes. The heat energy then dissipates to the surrounding tissues, thereby causing damage. Further, the high RF field may cause a high current to flow through the leads and within internal components of the IMD. As a result, the MRI system may cause the IMD to generate a voltage at the leads that then electrically excites the tissue. In certain instances, the voltage generated at the leads may induce fibrillation of the heart. The current induced by the RF field of the MRI system may also inhibit the output of pacing pulses to the patient.
Methods have been proposed to reduce the effects of interference by MRI systems on implantable medical devices. Some of these methods focus on reducing the effects of interference on the lead itself. Certain conventional leads have increased insulation surrounding the lead body, or have wires or conductors within the lead with reduced diameter to limit the effects of the RF fields. However, adding insulation or reducing the size of wires or conductors may increase the cost of the lead and may decrease the effectiveness of the IMD. Other known leads include a shield, such as a conductor or a wire braid, within portions of an insulating sheath surrounding the lead. However, conventional shield arrangements are unable to shield the pacing and/or sensing electrodes at the end of the lead. The electrodes remain unshielded, and thus are subject to excessive heating and the like.
A need remains for an improved, MRI-compatible, implantable lead that may be safely used during imaging with MRI systems without the generation of significant heat beyond safe temperature levels.